1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving a High Speed Shared Control Channel (HS-SCCH) in a Wideband Code Division Multiple Access (WCDMA) wireless communication system, and more particularly, to an apparatus and method for transmitting/receiving an ACKnowledgement/Negative ACKnowledgement (ACK/NACK) repetition factor over an HS-SCCH in a WCDMA wireless communication system.
2. Description of the Related Art
Mobile communication systems have evolved from a voice-based system into a high-speed, high-quality wireless packet data transmission system for providing data service and multimedia service. Standardization work dedicated to High Speed Downlink Packet Access (HSDPA) and Evolution-Data and Voice (1xEV-DV) primarily by the 3rd Generation Partnership Project (3GPP) and 3GPP2 committees is clear evidence of efforts to find a solution to 2 Mbps or higher-speed, high-quality wireless data packet transmission in the 3rd Generation (3G) mobile communication system.
In wireless communications, the radio channel environment is an obstacle to high-speed, high-quality data service. For example, the radio channel environment often varies due to a signal power change caused by fading, shadowing, Doppler effects caused by movement of the mobile station and frequent velocity changes of the mobile station, interference from other users, and multipath interference, as well as Additive White Gaussian Noise (AWGN). Thus, it follows that an advanced technology beyond the technologies of conventional 2nd Generation (2G) and 3G mobile communication systems is needed to improve adaptability to the channel changes in order to provide high-speed wireless data packet service. Although fast power control adopted in conventional systems improves adaptability to the channel changes, the 3GPP and 3GPP2 dedicated to standardization of a high-speed data packet transmission system has commonly adopted Adaptive Modulation and Coding (AMC) and Hybrid Automatic Repeat Request (HARQ).
AMC is a scheme for changing a modulation scheme and a coding rate adaptively according to the change of a downlink channel environment. Generally, a User Equipment (UE) measures the Signal-to-Noise Ratio (SNR) of a downlink signal and reports it to a Node B (a base station). The Node B then estimates the downlink channel environment based on the SNR information and determines an appropriate modulation scheme and a coding rate of a channel encoder according to the estimation.
HARQ is a scheme of retransmitting the packet to compensate for an error, when it occurs in an initially transmitted data packet. The HARQ scheme includes Chase Combining (CC), Full Incremental Redundancy (FIR), and Partial Incremental Redundancy (PIR). In CC, the same packet as initially transmitted is retransmitted. In FIR, instead of the same initially transmitted packet, a packet having only redundancy bits generated from a channel encoder is retransmitted. In PIR, a data packet comprised of information bits and new redundancy bits is retransmitted.
While AMC and HARQ are independent techniques to increase adaptability to the change of links, a combination of AMC and HARQ can improve the system performance considerably. That is, a transmitter in a Node B determines a modulation scheme and a coding rate for a channel encoder adaptively according to the downlink channel status and transmits a data packet correspondingly. A receiver in a UE, if it fails to decode the data packet, requests a retransmission. The Node B retransmits a predetermined data packet in a predetermined HARQ scheme in response to the retransmission request.
To support the above-described schemes, a UE and a Node B need to exchange related control signals. A control channel for delivering the related control signals in an HSDPA communication system is called a High Speed Shared Control Channel (HS-SCCH). That is, the HS-SCCH delivers control signals related to a High Speed Physical Downlink Shared Channel (HS-PDSCH) for transmitting user data at a high rate.
FIG. 1 illustrates the structures of an HS-SCCH and an HS-PDSCH in a conventional HSDPA communication system.
As illustrated in FIG. 1, an HS-SCCH 110 is transmitted two slots earlier than a HS-PDSCH 120, for delivering control information necessary for demodulation of the HS-PDSCH 120.
The types of control information for supporting the demodulation of the HS-PDSCH 120 are show in Table 1 below.
TABLE 11st part2nd part7-bit CCS information6-bit TB size information1-bit MS information3-bit HARQ Process ID3-bit RV information1-bit NI16-bit UE ID
The HS-SCCH 110 comprises three slots. The first slot delivers the CCS (Channelization Code Set) information and the MS (Modulation Scheme) information, while the second and third slots deliver the TB (Transport Block) size information, the HARQ Process ID, the RV (Redundancy and constellation Version) information, the NI (New data Indicator), and the UE ID (UE Identifier). The reason for dividing the HS-SCCH slots into two parts is to rapidly acquire the CCS information and the MS information that are important for demodulation of the HS-PDSCH 120.
The control information transmitted over the HS-SCCH is now described in detail.
1. CCS Information
The HSDPA communication system uses up to 15 Orthogonal Variable Spreading Factor (OVSF) codes with a Spreading Factor (SF) of 16, which serve as channelization codes. The CCS information indicates the number of channelization codes that are used to transmit the HS-PDSCH. The CCS information is 7-bit information as shown in Table 1. Using the CCS information, a UE acquires the type and number of channelization codes necessary for despreading.
FIG. 2 is a diagram illustrating an OVSF code tree in the conventional HSDPA communication system.
As illustrated in FIG. 2, each OVSF code (channelization code) is represented as C(i, j) according to its location in the code tree. The variables i and j of C(i, j) denote an SF and a location counted from the leftmost position in the OVSF code tree, respectively. For example, C (16, 0) refers to an OVSF code with an SF of 16 at the first location from the leftmost position in the OVSF code tree.
In FIG. 2, for SF=16, the 7th to 16th OVSF codes, C(16, 6) to C(16, 15) are assigned to the HS-PDCH for HSDPA service. A plurality of OVSF codes available for the HSDPA service can be code-multiplexed for a plurality of UEs at an identical time. A Node B determines the number of OVSF codes to be allocated to each UE and the locations of the allocated OVSF codes in the code tree, and transmits the determined number and positions to the UEs over the HS-SCCH using the CCS information
FIG. 3 is a diagram illustrating a CCS table for determining the number of codes used for transmission of an HS-PDSCH and the locations of the codes on a code tree in the conventional HSDPA communication system.
As illustrated in FIG. 3, a longitudinal axis index and a transverse axis index in the CCS table represent a 3-bit cluster code indicator and a 4-bit tree offset indicator, respectively. The cluster code indicator and the tree offset indictor are combined to constitute CCS information. In a unit square, an upper numeral “m” indicates the number of allocated codes and a lower numeral “Δ” indicates an offset from the left/right of the code tree. For example, “1/1” corresponds to allocation of one channelization code (i.e., the first channelization code) and “3/2” corresponds to allocation of three channelization codes (i.e., the second to fourth channelization codes).
A Node B and a UE both manage the CCS table illustrated in FIG. 3. Using the CCS table, the Node B organizes and transmits CCS information to the UE. Using the CCS information from the Node B, the UE accesses the CCS table to acquire allocated channelization codes (spreading codes).
2. MS Information
As described above, the AMC scheme adaptively changes a modulation scheme for a modulator and a coding rate for a channel encoder according to channel environments. When two modulation schemes of Quadrature Phase Shift Keying (QPSK) and 16-array Quadrature Amplitude Modulation (16QAM) are used, the Node B must inform the UE of the modulation scheme and coding rate of a current packet at each packet transmission. Because the coding rate is matched with information such as a TB set, an HS-PDSCH CCS, and an MS, the Node B has only to transmit the MS information to the UE.
3. TB Size Information
The TB size information indicates the size of a TB on a transport channel mapped to a physical channel.
4. HARQ Process ID (HAP)
HARQ is a special case of ARQ with the following two schemes introduced to increase transmission efficiency. One is to transmit a retransmission request and a response between a UE and a Node B and the other is to temporarily store data having errors and combine the data with retransmitted data at a receiver.
Meanwhile, a typical Stop And Wait (SAW) ARQ scheme allows transmission of the next packet data only when an ACK is received for the current packet data. In this case, even if the packet data can be transmitted, the ACK must be awaited.
An n-channel SAW ARQ provided to solve this problem allows transmission of successive packet data without receiving an ACK for the current packet data. That is, n time-divided logical channels are established between the UE and the Node B. The Node B informs the UE which logical channel delivers specific packet data using HARQ process information including a predetermined time slot or channel number. Using the HARQ process information from the Node B, the UE reorders in the original order packet data received at a particular point in time or soft-combines the packet data. Such HARQ process information is the HARQ process ID (HAP).
5. RV Information
Table 2 shows RV coding for 16QAM, while Table 3 shows RV coding for QPSK. The RV information includes parameters s, r and b; parameters s and r are values used for rate matching.
As shown in Table 4, parameter b is information about constellation rearrangement. A transmitter transmits a signal using one of four constellations shown in Table 4.
TABLE 2Xrv (value)srb01001000211130114101510261037110
TABLE 3Xrv (value)sr010100211301412502613703
TABLE 4bOutput bit sequenceOperation0s1, s2, s3, s4None1s3, s4, s1, s2Swapping MSBs with LSBs2s1, s2, s3, s4Inversion of the logical values of LSBs3s3, s4, s1, s21 & 2
6. NI
The NI indicates whether a current packet is initially transmitted or retransmitted. The NI is represented in one bit.
7. UE ID
The UE ID is specific to each UE. Using its UE ID The UE determines whether the HS-SCCH and the HS-PDSCH are allocated to it in each time slot.
The control information transmitted over the HS-SCCH is determined according to an ACK/NACK and a Channel Quality Indicator (CQI) that are fed back from a receiver. For example, when an ACK is fed back from the receiver and thus new packet data are transmitted, the NI is set to “NEW”. The MS information and the CCS information are determined using the CQI fed back from the receiver.
FIG. 4 is a diagram illustrating a procedure for exchanging the control information in the conventional HSDPA communication system.
As illustrated in FIG. 4, a transmitter transmits a new packet with the NI set to “NEW”. Also, using the Xrv, the transmitter informs a receiver of the parameters s, r and b used for the packet transmission.
Based on the control information from the transmitter, the receiver decodes a received packet to determine an ACK/NACK, and transmits the determination result to the transmitter over a High Speed Dedicated Physical Control Channel (HS-DPCCH).
When receiving a NACK from the receiver, the transmitter retransmits the corresponding packet with the NI set to “CONTINUE” and with the Xrv set to one of 0 to 7. On the other hand, when receiving an ACK from the receiver, the transmitter transmits a new packet with the NI set to “NEW” and with the Xrv set to one of 0 to 7.
In a predefined CQI feedback cycle (P-CQI), the receiver measures a CQI of a downlink channel to feed back the measured CQI to the transmitter. This feedback CQI is used to determine an MS and a CCS.
An ACK/NACK transmitted from a transmitter is now described in detail.
The existing 3GPP standard allows repeated transmission of an ACK/NACK up to four times. This repetition time is called an ACK/NACK repetition factor. A UE receives an HS-PDSCH packet and repeatedly transmits an ACK or a NACK by the ACK/NACK repetition factor according to whether the received packet has an error. For example, when the repetition factor is 4 and the Cyclic Redundancy Check (CRC) result on the received packet is an ACK, the UE repeatedly transmits an ACK four times during a corresponding Transmission Time Interval (TTI) and the subsequent three TTIs.
In the conventional communication system, the ACK/NACK repetition factor is transferred from a Radio Resource Control (RRC) layer of a UE to a physical layer of the UE, which is problematic because much delay is required to reflect the reception conditions in a Node B. Accordingly, is a scheme is needed for rapidly applying an ACK/NACK error or an ACK/NACK data loss, which is caused by an increased interference in an uplink and by the distortion of a radio channel, to the adjustment of the ACK/NACK repetition factor. For the rapid adjustment of the ACK/NACK repetition factor, it is desirable that the ACK/NACK repetition factor is adjusted by a Node B that can directly detect the channel conditions of an uplink, rather than by an UE. Therefore, what is required is a scheme for adjusting the ACK/NACK repetition factor at the Node B and rapidly transmitting the adjusted ACK/NACK repetition factor to the UE.